The present disclosure relates to hand-held and otherwise portable metal detectors such as are used for finding buried coins, historical artifacts, gold nuggets, unexploded ordnance or other metal objects. More particularly, the present disclosure relates to a quasi-resonant transmitter for a metal detector of the pulse induction type.
Metal detectors designed for portable use especially in the hand-held configuration are well known. Such metal detectors drive a varying electrical current through a transmit coil, the varying current producing a corresponding varying magnetic field. This varying magnetic field induces a varying voltage within the effective region of the field. The varying voltage induces a corresponding varying electrical current to flow in electrically conductive objects, for example, metal objects present in the field. The flow of current in the electrically conductive object distorts the field, and the resulting distortion of the field being detected electronically by receiving means incorporated into the metal detector. The presence of electrically conductive objects thus detected is indicated to the operator typically by audio and or visual means.
The principal technologies used in hand-held metal detectors are single-frequency sinusoidal induction balance commonly referred to as “VLF”, multiple frequency induction balance with rectangular voltage drive to the transmit coil, and pulse induction “PI”, the searchcoil of which may or may not be of the induction balance type. The features of the present disclosure relate to pulse induction technology.
Conventional pulse induction operates as a total loss system. The energy produced by the collapse of the magnetic field during flyback is dissipated in an ohmic resistance. The total-loss basis technology used in commercial hand-held pulse induction metal detectors requires heavy batteries, delivers short battery life, and exhibits sluggish response characteristics resulting from signal integration in the receiver in the quest to improve signal-to-noise ratio in order to have detection sensitivity comparable to ordinary VLF induction balance metal detectors. The apparatus and method of the present disclosure, in contrast, captures flyback energy and recycles it back to the transmit coil.
Corbyn GB Patent Document No. 2,071,327A discloses a pulse induction metal detector including means for balancing out the effects of magnetic viscosity so that metal targets can be detected when buried in magnetically viscous soils. Corbyn also discloses classic “pulse induction” transmitter waveforms. However, energy delivered to the transmit coil in Corbyn is dissipated either during the transmit on-time or during flyback, and no energy is recovered from the collapse of the magnetic field to be reused for subsequent energization of the transmit coil.
Johnson U.S. Pat. No. 4,868,504 discloses a pulse induction system which captures flyback energy by steering it into a DC power supply for later reuse. Johnson discloses a generally triangular-shaped transmit coil current waveform and the flyback voltage is clamped approximately to the DC power supply voltages. The present disclosure, in contrast, exhibits an approximately rectangular-shaped transmit current, and the flyback voltage is typically at least several times as great as the DC power supply voltage.
Candy U.S. Pat. No. 6,686,742 discloses a pulse induction system which captures flyback energy by steering it into a high voltage DC power supply. The coil current waveform of the flyback event is generally similar to that of a total loss system. A DC-DC converter then transfers the energy thus stored back to the main lower voltage DC power supply. The present invention, in contrast, does not have a high voltage DC power supply and does not return the stored energy to a lower voltage DC power supply.
The apparatus and method of the present disclosure provides power consumption efficiency typically several times greater than an otherwise comparable total loss system. The apparatus and method of the present disclosure generates an approximately rectangular coil current waveform providing detection of high conductivity or ferrous metal target objects better than of an approximately saw tooth triangular waveform of the same duration. The apparatus and method of the present disclosure also provides lower voltage stresses on components in the flyback circuit as compared to an otherwise similar total loss system terminating the same current. There is also less radiation of higher-order harmonics as compared to an otherwise similar total loss system and a relatively high rate of change of current at the end of flyback making possible the detection of low conductivity nonferrous metal target objects with good sensitivity.
An illustrated embodiment of the present disclosure includes capacitance added to the transmitter circuit of a pulse induction metal detector. The capacitance both stores energy from flyback at the end of the transmit pulse, and provides energy to initiate the transmit pulse. The amount of capacitance is not large, such as would be necessary to provide a DC voltage, rather it is small such that its resonant period with respect to the transmitter coil inductance is less than twice the nominal transmit period. The voltage waveform at the beginning and ending of the transmit pulse are approximately ¼ cosines. The resulting transmit current is approximately rectangular with approximately ¼ sine leading and trailing edges.
In an illustrated embodiment of the present disclosure, a method is provided for recycling flyback energy in a pulse induction metal detector. The method includes providing a pulsed magnetic field transmitting system including a DC voltage power source having a first end connected to a ground common node, an inductive transmitter coil configured to create a magnetic field for energizing metal objects to be detected by the metal detector, a capacitor having a first end connected to the common node, a first electronic switch connected between a second end of the capacitor and a first end of the coil, a second electronic switch connected between a second end of the coil and the common node, a first rectifier connected between the second end of the coil and the second end of capacitor, and a second rectifier connected between a second end of the power source and the first end of the coil. The first rectifier is adapted to steer electric current from the coil to the capacitor, and the second rectifier is adapted to steer electric current from the power source to the coil. The method also includes operating the transmitting system in a repeating sequence, in quasi-steady-state operation. The repeating sequence includes turning on the first switch and the second switch creating a path for current to flow from the capacitor through the coil to the common node thus discharging the capacitor; turning off the first switch with current continuing to flow in the coil from the power source through the second rectifier; turning off the second switch causing the current flowing through the coil to be steered through the first rectifier to the capacitor thus charging the capacitor; and providing a time interval without current flow in the coil during which eddy currents flowing in metal objects are detected by the metal detector of which the pulsed magnetic field transmitting system is a part.
In another illustrated embodiment of the present disclosure, a quasi-resonant transmitter circuit apparatus for a pulse induction metal detector apparatus is adapted to capture and recycle flyback energy and to transmit a pulsed magnetic field for energizing metal objects to be detected. The transmitter circuit apparatus includes a DC voltage power source having a first end connected to a ground common node; an inductive transmitter coil configured to create a magnetic field for energizing metal objects to be detected by the metal detector; and a capacitor having a first end connected to the common node. The apparatus also includes a first electronic switch connected between a second end of the capacitor and the first end of the coil; a second electronic switch connected between the second end of the coil and the common node; a first rectifier connected between the second end of the coil and the second end of capacitor; and a second rectifier connected between the second end of power source and the first end of the coil. The first rectifier is adapted to steer electric current from the coil to the capacitor, and the second rectifier is adapted to steer electric current from power source to the coil. The apparatus further includes timing means for controlling of an on state and an off state of the first and second switches and so that the transmitter circuit operates in a predetermined repeating sequence, the timing means turning on the first switch and the second switch to create a path for current to flow from the capacitor through the coil to the common node thus discharging the capacitor; then turning off the first switch with current continuing to flow in the coil from the power source through the second rectifier; then turning off the second switch causing the current flowing through the coil to be steered through the first rectifier to the capacitor thus charging the capacitor; and then maintaining the first and second switches in the off state providing a time interval without current flow in the coil during which eddy currents flowing in metal objects are detected by the metal detector of which the transmitter circuit is a part.
Additional features of the present invention will become more apparent to those skilled in the art upon consideration of the following detailed descriptions of illustrative embodiments exemplifying the best mode of carrying out the invention as presently perceived.